In my case it seems to be a security/codec issue with vista because I
can paste the url into the wmp "open url" window and it plays fine. I
simply can't click to play now.
Media Player like other things in Vista suffer performance wise
because of the idiotic changes Microsoft made to "improve" overall
security. Naturally they failed in that effort just like they failed
in how UAC is implemented. The best solution is use any of several
free media players that are superior to Media Player and have no such
issues.
I use all of the following for various purposes:
1. GOM Player
2. VLC Media Player
3. XnView
All the above are free, all work fine under Vista.